An “ad hoc network” refers to a self-configuring network of nodes connected by wireless links which form an arbitrary topology. An ad hoc network typically includes a number of geographically-distributed, potentially mobile units, sometimes referred to as “nodes,” which are wirelessly connected to each other by one or more links (e.g., radio frequency communication channels). The nodes can communicate with each other over a wireless media without the support of an infrastructure-based or wired network. Links or connections between these nodes can change dynamically in an arbitrary manner as existing nodes move within the ad hoc network, as new nodes join or enter the ad hoc network, or as existing nodes leave or exit the ad hoc network. One characteristic of the nodes is that each node can directly communicate over a short range with nodes which are a single “hop” away. Such nodes are sometimes referred to as “neighbor nodes.” A large network can be realized using intelligent access points (IAP) which provide wireless nodes with access to a wired backhaul.
A wireless mesh network is a collection of wireless nodes or devices organized in a decentralized manner to provide range extension by allowing nodes to be reached across multiple hops. In a multi-hop network, communication packets sent by a source node can be relayed through one or more intermediary nodes before reaching a destination node. When a node transmits packets to a destination node and the nodes are separated by more than one hop (e.g., the distance between two nodes exceeds the radio transmission range of the nodes, or a physical barrier is present between the nodes), the packets can be relayed via intermediate nodes (“multi-hopping”) until the packets reach the destination node. In such situations, each intermediate node routes the packets (e.g., data and control information) to the next node along the route, until the packets reach their final destination. For relaying packets to the next node, each node maintains routing information collected through communication with neighboring nodes. The routing information can also be periodically broadcast in the network to reflect the current network topology. Alternatively, to reduce the amount of information transmitted for maintaining accurate routing information, the network nodes may exchange routing information only when it is needed. In an approach known as Mesh Scalable Routing (MSR), nodes periodically send HELLO messages (e.g., once per second) that contain routing and metrics information associated with the route to its bound intelligent access point (IAP), and discover certain peer routes on-demand.
A key aspect for optimal performance of a mesh network is controlling and coordinating the transmissions from different devices to minimize interference and collisions among the devices of the mesh network. One method defined in the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard includes a collision avoidance feature known as request to send (RTS) and clear to send (CTS). (see: http://standards.ieee.org/getieee802/index.html or contact the IEEE at IEEE, 445 Hoes Lane, PO Box 1331, Piscataway, N.J. 08855-1331, USA) In this method, when RTS and CTS control signals are exchanged between a transmitter and a receiver, devices within range of the transmitter and the receiver defer from accessing the channel until the transmission is complete. This method is vulnerable to interference during the transmission of the RTS and CTS control signals and is not as effective when some transmitters in the network emit signals at a much lower power level than others. Additionally, it is generally not employed when the frames being transmitted (e.g. voice frames) are of short duration compared to the control signal.
Another method is defined in United States Patent Application Publication Number 20070060141 to Kangude et al, entitled “Mesh Deterministic Access,” which was published on Mar. 15, 2007. This application describes a method to coordinate the transmissions of mesh devices in a mesh network using a distributed schedule. A transmission scheduled is established between two mesh devices when the devices identify an unreserved period of time, called a Mesh Deterministic Access (MDA) opportunity (MDAOP), during which other mesh devices are not scheduled to transmit. Once median MDAOP is identified, both devices advertise the selected MDAOP, allowing nearby devices to avoid this period of time when making their own reservations. A device making a reservation as a transmitter must start and complete its transmissions during the reserved period of time. The transmitter uses a special set of contention parameters and a special channel access function to access the media during its MDAOP. The MDAOP owner is not allowed to use the special set of contention parameters nor the special channel access function outside its MDAOP. This method fails to address several practical issues including: how the mesh point (MPS) may transmit outside an MDAOP in the event of a busy channel, how the MP may retransmit outside the MDAOP, and how to protect the MP against transmissions from non-MDA devices.
United States Patent Application Publication Number 20040264397, to Benveniste et al, entitled “Power-saving mechanism for periodic traffic streams in wireless local-area networks,” published on Dec. 30, 2004, teaches a method of establishing a periodic schedule between an access point and one or more of the stations associated with the access point. Once a schedule is established, both the access point and the station buffer traffic for the each other until the scheduled transmission period arrives, and only then may both begin to transmit. This method is not effective when more than one access point exists in the network because it does not teach how to coordinate the scheduling operation between access points.
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